Multi-layer interference coatings

ABSTRACT

A multi layer interference coating comprising at least one multi layer stack deposited on a reflective layer ( 9 ), wherein each multi layer stack comprises a first dielectric layer ( 11 ), a layer of asbsorbing material ( 10 ) and a second layer of dielectric material ( 11 ) arranged in series and having a reflectance spectrum in the infrared region comprising at least one maximum. The dielectric layers are of equal optical thickness and typically are of the same material. The multi layer structure of the coating is such that incident electromagnetic radition, for which odd multiples of half wavelengths correspond to the optical thickness of the multi layer coating at the incident wavelength do not propagate within the coating and reflection at these wavelengths, is suppressed. Coatings may therefore be designed to have a near satured color in the visible wavelength spectrum. The reflective layer may be a metal or a conducting oxide, a conducting nitride, a conducting silicide or a conducting sulphide. The absorbing layers may be Cr, V, Pd, Ni, Pt, conducting oxides, or substoichiometric metal oxides, such as TiO x . In one form of the coating, where a non-metal absorber is used, the second dielectric layer may be removed in at least one of the multi layer stacks. The coating may be incorporated in a system for verifying the authenticity of an article.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to multi layer coatings which use interferenceeffects rather than absorptive dyes to modulate spectral reflectances.The coatings may be in the form of a thin film or a pigment and may beapplied to products or articles for anticounterfeiting purposes in orderto identify genuine goods. The coatings may also be used for spectralcontrol of thermal emittance or for thermal control purposes.

2. Discussion of Prior Art

The use of pigments which use interference effects to provide colour isbecoming increasingly popular in many areas. Absorptive colour pigmentsare often based on toxic heavy metals which can be problematic.Interference colours have the further advantage that they may be angletuned to provide additional decorative effects. Furthermore, once thematerial system has been qualified, new colours may be produced withoutthe need for extensive weathering trials.

Optically variable pigments (OVPs) based on interference effects andhaving a metal-dielectric-metal layer structure (M′-D-M) are known inthe prior art (U.S. Pat. No. 4,705,356). The structure comprises areflective metal layer (M′) a dielectric layer (D) and a thin absorbingmetal layer (M), which forms a Fabry Perot cavity. The OVP has a largecolour shift with viewing angle which makes it difficult forcounterfeiters to reproduce by other means. Structures based on multipleperiods of dielectric spacer-metal absorber layer pairs and constructedon reflective metal layers (e.g. M′(D)^(n)) are also known (U.S. Pat.No. 5,214,530). The structures are peak suppressing (i.e. reflectionminima are suppressed) for the purposes of producing stronger chromaticeffects.

Relevant background to the present invention can also be found in U.S.Pat. No. 5,437,931 which relates to optically variable multi layer filmsproviding reflection characteristics in the visible wavelength region.

SUMMARY OF THE INVENTION

The present invention specifically relates to multi layer interferencecoatings which have strong reflectance characteristics in the infraredwavelength region. The structures are peak suppressing but have theadvantage over known multi layer structures that they comprise fewerlayers making fabrication more straightforward. Furthermore, in one formof the invention, fabrication is not only easier in this respect butalso because of the particular materials used.

There are a number of applications for coatings exhibiting strongreflectance characteristics in the infrared wavelength region. Inparticular, for covert marking and anticounterfeiting applications,hidden spectral features may be used to uniquely identify an article orproduct. Conventional multi layer interference structures, however, donot operate well in the infrared wavelength region. The coatings may beconstructed such that they have strong reflectance characteristics inthe infrared.

The invention also relates to an anticounterfeiting or product trackingsystem in which the multi layer coatings may be incorporated, theoperation of which may be covert.

According to the present invention, a multi layer interference coating,having a reflectance spectrum in the infrared wavelength regioncomprising at least one maximum, comprises;

a reflective layer having at least one surface for carrying one or moremulti layer stacks,

wherein each multi layer stack comprises a first layer of dielectricmaterial, a layer of absorbing material and a second layer of dielectricmaterial arranged in series with the layer of absorbing materialsituated between the first and second layers of dielectric material,

wherein the second layer of dielectric material has substantially thesame optical thickness as the first layer of dielectric material at awavelength substantially corresponding to a maximum in the reflectancespectrum and wherein the layer of absorbing material has a refractiveindex n and an optical constant k,

such that incident electromagnetic radiation, having a wavelength atwhich odd multiples of half wavelengths substantially correspond to theoptical thickness of the coating at said wavelength, is substantiallyabsorbed within the coating.

Preferably, the first dielectric material is the same as the seconddielectric material. At least one of the first or second dielectricmaterials may be any one of titanium oxide (TiO₂), magnesium fluoride(MgF₂), zinc sulphide (ZnS), zinc selenide (ZnSe), silicon (Si),germanium (Ge) or barium fluoride (BaF₂)

In a preferred embodiment, the n/k ratio of the absorbing material isbetween 0.7 and 1.3, and is preferably substantially equal to 1, in theinfrared wavelength region.

The reflective layer may be a metal, for example, gold, silver oraluminium. The absorbing material may be a metal, for example, chrome(Cr), vanadium (V), palladium (Pd), nickel (Ni) or platinum (Pt).

In another embodiment of the invention, the absorbing material may be asubstoichiometric metal oxide. Preferably, the substoichiometric metaloxide may be of the same material as the layer of dielectric material.For example, the substoichiometric metal oxide may be titanium oxide(TiO_(x)) and the dielectric material may be titanium dioxide (TiO₂).

In another form of the invention, the absorbing material may be aconducting oxide, a conducting nitride or a conducting silicide, forwhich the n/k ratio is substantially equal to 1 in the infraredwavelength region. For example, the absorbing material may be indium tinoxide (ITO), doped tin oxide, for example SnO₂:F, or titanium nitrate,(TiN). Alternatively, the absorbing material may be vanadium dioxide(VO₂) substoichiometric vanadium oxide (VO_(2−x)) or doped VO₂ and thereflectance spectrum of the coating may be varied with temperature.

If a conducting oxide, nitride, silicide or sulphide is used as theabsorbing material, it may be advantageous to use a like conductingoxide, nitride or silicide as the reflective substrate. Alternatively,the reflective layer may be a metal, such as gold, silver or aluminium.

The reflective layer may have two opposite facing surfaces wherein atleast one multi layer stack is deposited on each of the two oppositefacing surfaces such that the coating has a substantially symmetricstructure about the reflective layer.

The reflective layer may comprise a reflective material deposited on anon-reflecting particulate substrate or may be a reflecting particulatesubstrate.

The coating may be in the form of a thin film which may be flaked intofragments and incorporated into a paint or ink. Alternatively, thereflective layer may be substantially spherical, with at least one multilayer stack is deposited on the substantially spherical reflectivelayer. The substantially spherical multi layer structure may then beincorporated into a paint or ink.

The coating may be applied directly to the surface of an article orapplied to a label to be applied to an article. Alternatively, thecoating may be incorporated into a moulded article.

In another embodiment of the invention, where the absorbing material isa non-metal material, the second layer of dielectric material may beabsent in at least one of the multi layer stacks,

such that incident electromagnetic radiation, having a wavelength atwhich odd multiples of quarter wavelengths substantially correspond tothe optical thickness of the coating at said wavelength, issubstantially absorbed within the coating.

In this form, the absorbing material may be a conducting oxide, nitride,silicide or sulphide, for example ITO, doped tin oxide (for exampleSnO₂:F), TiN, VO₂, substoichiometric VO₂ (VO_(2−x)) or doped VO₂.Alternatively, the absorbing material may be a substoichiometric metaloxide, for example TiO_(x).

According to another aspect of the invention, a system for marking anarticle and checking its authenticity comprises;

a multi layer interference coating, having a reflectance spectrumcomprising at least one maximum, wherein the coating is applied to thearticle to be authenticated,

means for illuminating the coating with incident radiation comprisingone or more wavelengths wherein one or more of the wavelengthssubstantially correspond to a maximum or a minimum in the reflectancespectrum of the coating and

means for detecting radiation reflected from the coating atsubstantially one or more of the wavelengths,

whereby the detection of the reflected radiation provides an indicationof the authenticity of the article.

A comparison of the reflected radiation at two or more wavelengths mayprovide an indication of the authenticity of the article.

According to another aspect of the invention, a system for covertlymarking an article and checking its authenticity comprises;

a multi layer interference coating having a temperature dependentreflectance spectrum, wherein the coating is applied to the article tobe authenticated,

means for varying the temperature of the coating such that thereflectance of the coating at one or more wavelengths may be varied asthe temperature is varied,

means for illuminating the coating with infrared radiation comprisingone or more wavelengths substantially corresponding to one or more ofthe wavelengths at which the reflectance varies and

means for detecting infrared radiation reflected from the coating at oneor more of the wavelengths at which the reflectance varies,

whereby a comparison of the reflected radiation before and after thetemperature of the coating is varied provides an indication of theauthenticity of the article.

A single laser may be used to both illuminate and vary the temperatureof the coating. A thermal imager or a spectrophotometer may be used todetect the reflected radiation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by example only, with reference tothe following figures in which;

FIG. 1 shows a diagram of the conventional metal-dielectric-metal(M′-D-M) structure of an optically variable pigment,

FIG. 2 shows the reflectance spectrum of a single layer of dielectric ona silver reflector,

FIG. 3 shows the reflectance spectrum of the M′-D-M structure shown inFIG. 1,

FIG. 4 shows the electric field intensity propagating within the M′-D-Mstructure shown in FIG. 1,

FIG. 5a shows an asymmetric reflector-dielectric-absorber-dielectricstructure (R-D-A-D),

FIG. 5b shows a symmetric reflector-dielectric-absorber-dielectricstructure, based around a central reflecting layer (D-A-D-R-D-A-D),

FIG. 6 shows the reflectance spectrum of an R-D-A-D structure,

FIG. 7 shows the electric field intensity in the R-D-A-D structure ofFIG. 6 for an odd multiple of half-waves corresponding to the maximum inthe reflection spectrum,

FIG. 8 shows the electric field intensity in the R-D-A-D structure ofFIG. 6 for an even multiple of half-waves corresponding to the maximumin the reflection spectrum,

FIGS. 9 and 10 show the colour trajectories for M′-D-M and R-D-A-Dstructures respectively, overlaid on a 1931 CIE chromaticity diagram,

FIG. 11 shows the reflectance spectrum of an R-D-A-D with areflector-dielectric-substoichiometric metal oxide-dielectric structure,

FIG. 12 shows the reflectance spectrum of areflector-dielectric-substoichiometric metal oxide structure,

FIG. 13 shows the reflectance spectrum of areflector-dielectric-conducting oxide-metal structure, incorporating alayer of indium tin oxide,

FIG. 14a shows the reflectance spectrum of areflector-dielectric-conducting oxide-metal structure incorporating alayer of vanadium oxide (VO₂),

FIG. 14b shows the reflectance spectrum of areflector-dielectric-conducting oxide-metal structure incorporating alayer of substoichiometric vanadium oxide (VO_(2−x)) and

FIG. 15 shows a system incorporating the coating of the presentinvention which may be used to verify the authenticity of an article.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, a conventional metal-dielectric-metal (M′-D-M)structure comprises a metal reflector substrate 1, such as silver, adielectric layer 2 and a top layer of metal absorber 3.

The reflectance spectrum of a dielectric layer (for example magnesiumfluoride, MgF₂) deposited on a metal reflector substrate is shown inFIG. 2 and FIG. 3 shows the reflectance spectrum of the M′-D-M structureillustrated in FIG. 1 (i.e. including the metal top absorber layer 3).

The thin metal absorber layer 3 forms a Fabry Perot cavity and themetal/dielectric structure 2,3 acts as an induced absorber as nodes inan incident propagating electric field 4 that intersect with the surfaceare absorbed. This occurs when odd multiples of quarter-waves of theelectric field, corresponding to minima in the reflectance curve 6,propagate within the cavity. The minima 5 in the reflectance curve 6 aretherefore pulled down.

Even multiples of quarter-waves of the electric field (i.e. wholehalf-waves) with antinodes at the surface are not affected and so thereis an enhanced contrast between reflection minima 4 and reflectionmaxima 7, compared to the reflectance spectrum of a simplemetal-dielectric structure (FIG. 2). The visual result is that strongerreflection colours are seen by a human observer.

FIG. 4 shows the electric field intensity 8 propagating within theM′-D-M structure at a wavelength of 625 nm, corresponding to an oddmultiple of half waves and a maximum 7 b in the reflection curve 6 (seeFIG. 3). Odd multiples of half-waves (as shown in the figure) and evenmultiples of half-waves can therefore propagate in the structure. Themetal and dielectric regions 1,2,3 are indicated on the intensityspectrum.

FIG. 5 shows a reflector-dielectric-absorber-dielectric structure(R-D-A-D). The structure comprises a reflective metal layer 9 and a thinabsorbing layer 10 contained within a dielectric layer 11, such thatthere is a substantially equal optical thickness of dielectric 11 eitherside of the absorbing layer 10. For example, the reflective layer may besilver, the dielectric layer may be MgF₂ or TiO₂, and the absorber layermay be a metal such as Cr, V, Pd or Pt. The characteristics of suitable‘grey’ metals which may be used as the absorber layer are described inU.S. Pat. No. 4 705 356.

The reflectance spectrum of an R-D-A-D structure is shown in FIG. 6. Thepresence of the absorbing layer 10 in the middle of the dielectric layer11 has the effect of enhancing the contrast in the reflection spectrum.In this example, the reflective layer 9 is silver and has a thickness of50 mm, although this may be any thickness sufficient to ensure opacity.The dielectric material 11 in each layer is MgF₂, with each layer havinga thickness of 320 nm and the absorber layer 10 is Cr and has athickness of 6 nm. Comparing with FIGS. 2 and 3, the transmissionmaximum 7 b centred at approximately 620 nm has been suppressed leaving,in this example, only one discrete reflection maximum 12 occurring inthe visible wavelength region. In the case of a metal absorber, such asCr, the thickness for maximum contrast in the reflectance spectrum istypically, although not exclusively, between 6 and 15 nm depending onthe particular metal used and the refractive index of the dielectricmaterial.

The thickness of the dielectric layer determines the position at whichreflection minima and maxima occur in the reflectance spectrum.Typically, the dielectric layer may have a thickness, x, of between 10nm and 800 nm, although eventually, any effects visible to the human eyeare lost with increasing thickness as the number of reflectance maximaand minim in the visible region increases. However, applications such ascovert product marking may make use of complex reflectance profiles thatthe human eye cannot identify.

FIG. 7 shows the electric field intensity in the R-D-A-D structure foran odd multiple of half waves corresponding to the reflection maximum 12(as shown in FIG. 6). By placing an absorber layer 10 at the midpoint ofthe dielectric 11, odd multiples of half-waves corresponding to thereflection maximum 12 are suppressed. In the absence of the metalabsorbing layer 10, the electric field would propagate as shown in FIG.4, but induced absorption in the metal 10 suppresses propagation of themode.

FIG. 8 shows the electric field intensity in the R-D-A-D structure foran even multiple of half waves corresponding to the reflection maximum12. An antinode 13 in the propagating electric field 14 is coincidentwith the absorber layer and propagation is therefore allowed. The resultis therefore the suppression of propagating light modes within thestructure which are odd multiples of half waves.

Referring to FIG. 7, the effect of losing every other reflectance peakin the reflectance spectrum is to greatly enhance the chromaticity ofthe structure. The peaks that remain in the reflectance spectrum arewidely separated and so the purity of colour of each reflectance peak isnot diluted by the immediately adjacent peaks.

The enhanced chromaticity of the R-D-A-D structure is illustrated bycomparing FIGS. 9 and 10 which show colour trajectories for the M′-D-Mstructure and the R-D-A-D structure respectively. The colourtrajectories are realised by increasing the thickness of the dielectriclayers from 10 nm to 800 nm and overlaid on 1931 CIE chromaticitydiagram.

It can be seen from the chromaticity diagram that, for the R-D-A-Dstructure, the colour space available is almost fully saturated in theblue/purple colour region (15, FIG. 10). The green region available inthe R-D-A-D structure is also more extensive (16, FIG. 10), as is theavailable red region (17, FIG. 10).

U.S. Pat. No. 5,214,530 refers to a structure that gives peaksuppression for enhanced chromaticity effects but comprises a metalreflector and multiple periods of dielectric spacer and metal absorberlayers. The proposed R-D-A-D structures produce highly effective peaksuppression effects but require fewer layers. This benefits thefabrication process and lowers the cost of production.

The structure of the pigment is such that the a colour shift occurs asthe relative viewing angle is varied. If the object to which the coatingis applied is tilted in front of the observer, or the observer tiltstheir head, the coating therefore appears to change colour. By using adielectric material with a low refractive index, such as MgF₂, thestructure gives a stronger colour shift with angle. This may be used fordecorative effect or may be used as an anticounterfeiting measure, toprovide means for identifying a genuine article. If an equivalent coloureffect is required using a physically thinner dielectric layers, withminimal angle tuning effect, a higher refractive index dielectricmaterial such as titanium oxide (TiO₂) can be used to minimisesensitivity to viewing angle.

The coatings may be in the form of a continuous film, where the metaland dielectric layers are deposited on a substrate, such as plastic,which can then be applied to an article or a label. Alternatively, thepigment may be produced in flake form for subsequent incorporation intopaints, inks, polymer binder or moulded articles. Flakes can be preparedby depositing layers of metal and dielectric onto a disposablesubstrate, such as plastic sheet or continuous plastic roll, usingconventional vacuum based deposition techniques such as sputtering,thermal or electron beam evaporation or activated chemical vapourdeposition (CVD).

Preferably, in flake form, the structure deposited is symmetrical abouta central reflective layer so that flakes have the same appearanceregardless of their orientation. For example, the coating may be of theform D-A-D-R-D-A-D, as shown in FIG. 5b. An alternative way of formingthe flake is to take pre-existing flake substance, such as mineral ormica flakes, and deposit on a disposable substrate using, for example,known fluidised bed chemical vapour deposition, vacuum evaporation orsputtering techniques. Particulate substrates on which the layers aredeposited may be reflecting, in which case the particulate substrateforms the reflective layer. Alternatively, a reflective layer may bedeposited on a non-reflecting particulate substrate.

A spherical substrate may be used, such as a ceramic microsphere, andthe reflector, dielectric and absorber materials are layered onto thesphere using, for example, fluidised bed chemical vapour deposition,vacuum evaporation or sputtering techniques.

In a structure having the general form ofreflector-dielectric-absorber-dielectric (R-D-A-D), the choice ofmaterial for use of the absorbing layer is restricted to those whichhave a n/k ratio of approximately 1, where n is the refractive index ofthe material and k is the optical constant. The only metals currentlyknown to meet this criterion are ‘grey’ metals, including chrome (Cr),vanadium (V), nickel (Ni), palladium (Pd) or platinum (Pt).

FIG. 11 shows the reflectance spectrum of a R-D-A-D structure in whichthe absorber layer is a substoichiometric metal oxide (SMO) material. Inthis example, the reflective layer is aluminium, the dielectric layersare TiO₂ (thickness of each layer=165 nm) and the metal oxide layer isTiO_(x) (thickness=40 nm). In the visible wavelength region inparticular, the values of the n/k ratio for TiO_(x) are sufficient forthe material to be used as an effective absorber layer in such astructure. The TiO_(x) absorber layer suppresses propagation of oddmultiples of half waves, resulting in the suppression of every otherpeak in the spectrum (corresponding to the minima at 370 nm (18) and 530nm (19)). The reflectance peaks in the spectrum 20 gives the structure astrong purple colour.

In the example shown, a reflectance maximum 20 occurs at approximately440 nm, the short wavelength end of the visible spectrum. In such adevice, the thickness of the dielectric layers determines the positionat which reflection minima and maxima occur. Typically, the dielectriclayers may have a thickness, x, of between 10 nm and 800 nm. The metaloxide absorber layer is typically between 10-60 nm, depending on theexact value of the optical loss, k, and refractive index, n, of thedielectric material. By varying the materials and thickness of the layerstructure, the wavelengths at which the reflectance peaks occur cantherefore be shifted, not only within the visible spectrum, but alsointo the ultra violet or near infrared wavelength regions.

Further layers of dielectric material and substoichiometric metal oxidemay be included in the structures to give varying reflectancecharacteristics (e.g. R-D-SMO-D-SMO or R-D-SMO-D-SMO-D), althoughincreasing the number of layers increases the complexity of thefabrication process.

One advantage of using a substoichiometric metal oxide rather than ametal absorbing layer is in terms of the fabrication process as only onedeposition technique and apparatus is required to fabricate thedielectric and absorber layers. In conventional M′-D-M structures, inwhich the absorber layer is a metal, separate deposition sources arenecessary to prepare the dielectric and metal absorber layers.

Many deposition techniques that can produce thin film oxides require theinput of additional oxygen to produce materials in the maximum valencestates. Reactive sputtering using a titanium metal target andargon/oxygen (Ar/O₂) plasma can produce stoichiometric transparent TiO₂.Reducing the amount of oxygen injected into the plasma results in thedeposition of substoichiometric material. Using this technique, theswitch between layers of TiO₂ and TiO_(x) can be readily achieved bysimply modulating the oxygen flow.

It is possible that other techniques, such as electron beam deposition,may also be able to rely on supplies of additional oxygen to counter theloss of oxygen from solid source materials which may themselves besubstoichiometric TiO_(x). Conventional electron beam depositiontechniques may therefore be adapted to switch between the production ofTiO_(x)/TiO₂ layers.

Conventional chemical vapour deposition (CVD) using thermaldecomposition of titanium isopropoxide Ti(OC₃H₇)₄ results in thedeposition of semi opaque TiO_(x), although material produced using themethod does not have a high enough value of k to act as an absorber inthe manner described. Deposition in a reducing atmosphere, such as aAr/H₂ or CO/CO₂ mix, however, may remove enough oxygen to producematerial with the desired optical properties.

The coating of individual particles with oxide/suboxide layers may alsobe used to prepare such structures using fluidised bed techniques. Thefluidised bed approach utilises conventional CVD techniques and enablesthe coating of suitable substrate particles, for example metal or metalcoated flake or microspheres, with the oxide and sub-oxide layers. Thetechnique of coating individual particles using such a technique wouldbe conventional to one skilled in the art.

A coating having a structure of the formreflector-dielectric-substoichiometric metal oxide is therefore moreeasily fabricated than a conventional M′-D-M structure. FIG. 12 showsthe reflectance spectrum of a reflector-dielectric-substoichiometricmetal oxide structure wherein the metal absorbing layer used in aconventional M′-D-M is replaced with a substoichiometric metal oxidematerial. In this example the dielectric layer is TiO₂, having athickness of 220 nm, and the metal oxide layer is TiO_(x), having athickness of 20 nm. As in the previous example, any metal oxide whichcan be produced in substoichiometric form and has suitable opticalconstants may be used as the absorbing layer.

Referring to FIG. 12, the presence of the substoichiometric (or metalrich) metal oxide absorbing layer suppresses propagation of modesconsisting of odd multiples of quarter waves and increases thereflection contrast. In the absence of the metal oxide absorber, thereflectance at the minima would be considerably higher. In this example,the maximum in the reflectance spectrum 21 gives rise to a strongblue/green colour.

According to another aspect of the invention, conducting oxide materialsmay be used as the absorbing layer with the intention of producingstrong reflectance features in the infrared wavelength region. Inparticular, coatings exhibiting reflectance contrast in the infraredhave covert marking and anticounterfeiting applications, where hiddenfeatures can be used to identify an article or a product. Although thisrequirement can be met with dielectric coatings, these have to be fairlycomplex to produce the required spectral profile. Using the presentinvention, the same effect can be achieved with a three or four layerstructure, such as a reflector-dielectric-conducting metaloxide-dielectric structure.

The use of ‘grey’ metals, such as chrome and vanadium, as the absorbinglayer in an interference structure does not produce coatings that worksufficiently well in the infrared wavelength region. These materialsexhibit typical metallic behaviour in that the optical loss, k,increases rapidly in the infrared, taking the n/k ratio well away fromthe optimum ratio for such devices of 1.

Whereas grey metals tend to have n/k as unity in the visible wavelengthband, conducting oxides tend to reach this condition in the infrared,some having very low values of k (high levels of transparency) in thevisible region. The conducting oxides that are proposed for use asabsorbing materials in the present invention may be classified as eitheractive or non-active materials. Non-active refers to materials havingfixed spectral properties under external stimulus, whereas activematerials exhibit a dramatic change in spectral properties in responseto an external stimulus, such as heat.

In the case of non-active coatings, the absorber layer may be fabricatedusing a conducting oxide film, for example ITO. ITO is transparent inthe visible wavelength band, but has an increasing value of k in theinfrared wavelength band, to the point where it will effectively act asan absorbing layer in this application.

The reflectance spectrum of a reflector-dielectric-conductingoxide-dielectric coating is shown in FIG. 13. In this example thecoating is of the form shown in FIG. 5a and the structure comprises asilver back reflector and an ITO layer (thickness=60 nm) sandwichedbetween two dielectric (MgF₂) layers, each 1100 nm thick. The choice ofmaterial for the back reflector is wide and is simply restricted to anymaterial which will give good reflectivity over the range of interest.Suitable metal materials are gold, silver and aluminium. The requirementfor the dielectric material is that it needs to be transparent over therange of interest. Suitable materials for use in multi layerinterference structures for use in the infrared are magnesium fluoride(MgF₂), zinc sulphide (ZnS), zinc selenide (ZnSe), silicon (Si),germanium (Ge) and barium fluoride (BaF₂).

Referring to FIG. 13, curve 22 represents the reflectance when the ITOlayer is absent (i.e. there is a single dielectric layer of 2200 nm) andcurve 23 represents the reflectance when the ITO is present. In theabsence of the ITO layer, the visible spectral region is relativelyfeatureless and would appear to an observer to be an ordinary metalliccoating (if the reflective layer were a metal). However, the ITO layeracts as an absorber and gives rise to reflection maxima and minima inthe reflectance spectrum in the infrared wavelength region. Otherconducting oxides with appropriate values of k, for example doped tinoxide (e.g. SnO₂:F), may also be used to provide the required absorptionin this region. The number and positions of the reflectance peaks can bevaried by choosing different dielectric materials of varying thickness.

Non oxide conducting materials, such as titanium nitride (TiN) and ironsulphide (FeS₂) and metal silicides such as titanium silicide (TiSi),tantalum sulicide (TaSi) and tungsten silicide (WSi), may haveappropriate characteristics to be used as the absorber layer. Conductingoxides, nitrides, suicides and sulphides when deposited in sufficientthicknesses, can exhibit high reflectances in the infrared wavelengthregion, whilst, in the case of conducting oxides, maintaining hightransparency in the visible waveband. The materials may therefore alsobe used to form the reflective layer in the structures. Although thereflectance levels are somewhat lower than for metals, for certainapplications the reflectance may be sufficient. For example, asconducting oxides are transparent in the visible region, it may bebeneficial to use them in covert product marking applications.Furthermore, only two materials are then needed in total to fabricatethe coating; the conducting oxide, silicide or nitride material and thedielectric.

A layer of conducting oxide may also be used as the absorber layer(and/or as the reflective layer, as discussed previously) in a structurecomprising a single layer of dielectric material (areflector-dielectric-conducting oxide structure) to give strongreflectance characteristics in the infrared region.

A layer of VO₂ may also be used as the absorber layer in the multi layerinterference structures. VO₂ undergoes a reversible phase change from asemiconducting to a metallic state at 68° C., in the case of undopedstoichiometric material. In terms of optical properties, the phasechange manifests itself primarily in a large increase in k in theinfrared wavelength region. For a layer of sufficient thickness(approximately 300 nm) an increase in infrared reflection occurs withthe onset of metallic behaviour. When the phase change occurs, the n/kratio becomes very close to the n/k=1 ideal condition. VO₂ may thereforebe used as an absorber in the structures described previously, toproduce a structure having temperature dependent reflectioncharacteristics.

At low temperatures, a multi layer structure comprising a VO₂ absorbinglayer displays very little reflectance contrast in the infrared as thevalue of k is too low. Only a thin layer of VO₂ is required to producethe required active response the coating appears neutral in colour tothe human eye, as the reflectance spectrum is relatively featureless inthe visible wavelength region, and has the appearance of a typical metalcoating (if a metal reflector is used). On heating the structure, noeffect is observed in the visible region and the operation of thecoating is therefore covert. In the infrared wavelength region, theeffect would be the sudden appearance of reflectance contrast as the nowabsorbing VO₂ layer suppresses the propagation of certain multiples ofquarter or half waves.

The effect is illustrated in FIG. 14a which shows the reflectancespectrum in the infrared wavelength region for an activereflector-dielectric-VO₂-dielectric structure. In this example, thereflective layer is silver, the dielectric layers are MgF₂ (each layer1100 nm in thickness) and the VO₂ layer approximately 30 nm thick.Curves 24 and 25 represent the reflectance of the structure below andabove respectively the transition temperature of VO₂. This illustratesthat, upon heating the structure, a very high (>90% ) reflectancecontrast can be achieved between a maximum 26 and minimum 27 in thereflectance curve 25.

In order to achieve such a high reflectance contrast with a single layerof VO₂, a thickness of approximately 300 nm is required. Byincorporating a VO₂ layer into a multi layerreflector-dielectric-VO₂-dielectric structure, a high reflectancecontrast is achieved using a single VO₂ layer of only 30 nm thickness.As the VO₂ layer is the major cost factor, the reduction in thicknessoutweighs the cost of the extra dielectric layers.

Furthermore, VO₂ has a distinct brown appearance when deposited in asubstantial thickness. However, the appearance of an ordinary metalliccoating could be presented with the very thin layers necessary in thesedevices preserving the appearance of the underlying metal reflector (forexample, silver or aluminium). Such a coating would therefore besuitable as an anticounterfeiting measure and may be used, for example,to form features like the metallic thread in bank notes withoutdeviating from the inconspicuous metal appearance. If a visibly darkcoating were required, a material such as diamond like carbon may beused as the dielectric to hide the presence of the metal underlayer inthe visible wavelength region.

It is known that VO₂ may be doped with a transition metal, such astungsten and molybdenum. By using a doped VO₂ absorber layer acontinuously varying transition with temperature may be achieved,therefore giving a “greyscale” effect. This may also be achieved usingan absorbing layer of substoichiometric (metal rich) VO₂ (VO_(2−x)).FIG. 14b shows the reflectance spectrum of areflector-dielectric-VO_(2−x)-dielectric structure for three differenttemperatures, with curves 30,31,32 representing the reflectance at threedifferent increasing temperatures respectively. As the temperature isincreased, the reflectance increases in a continuous way, rather thanthe abrupt transition illustrated in FIG. 14a. Thus, a continuousvariation in reflectance is obtained as the temperature of the structureis increased.

Reflector-dielectric-VO₂ structures may also be used to provide activecoatings having high reflectance contrast in the infrared wavelengthregion, wherein the coating is activated by heating to a temperaturegreater than the transition temperature of VO₂.

In order to incorporate the coatings in an anticounterfeiting system,suitable illumination and detection means are required. FIG. 15 shows adiagram of the apparatus which may be used to verify the authenticity ofa product or article to which the coating is applied. This apparatus maybe used in conjunction with a coating having the form of any of thestructures to which the patent application relates, although in practicecoatings with strong reflectance characteristics in the visible regionit may be preferable to identify the coating simply by the observationof a change in colour upon tilting the angle relative viewing angle.

For example, referring to FIG. 15, the coating 40 is applied to anarticle 41 to be identified. The article is then illuminated withradiation 42 a from a source 43 and radiation 42 b reflected from thecoating 40 is detected by suitable detection means 44. Referring to theexample coating shown in FIG. 13, if the coating 40 is illuminated withradiation 42 a having a wavelength substantially corresponding to areflection maximum (45 or 46), the detection of reflected infraredradiation could be used to provide an indication of the authenticity ofthe article 41. The detection means should be sensitive to a wavelengthregion comprising the wavelength of the illuminating radiation. Inaddition, an actual measure of the intensity of reflected radiation 42 bmay be required to authenticate the article.

In an alternative embodiment the system may be used to verify theauthenticity of an article by illuminating the product to which thecoating is applied with radiation of two discrete wavelengths, onecoincident with a reflection maximum and one coincident with areflection minimum. This may be of particular use for a system in whichan active coating is incorporated. For example, referring to FIG. 14,before the structure is heated the intensity of radiation reflected atthe two wavelengths 47 and 48 will be similar. After the structure isheated, the reflected radiation at the two wavelengths will beconsiderably different. The difference between the reflectance at thetwo wavelengths would provide a more accurate measurement than ameasurement at a single wavelength. Although it may be preferable tomeasure the reflectance at wavelengths corresponding to a maximum andminimum in the reflectance spectrum, in principle any two wavelengthsmay be chosen for which there is a difference in the reflectance beforeand after the structure is heated.

A suitable source of radiation 43 for use in the apparatus may be a CO₂laser, an infrared HeNe laser or a solid state diode laser. In the caseof active coatings, the radiation source may also provide the heatingnecessary to activate the coating.

In the case of active reflector-dielectric-VO₂-dielectric coatings,including reflector-dielectric-VO_(2−x)-dielectric coatings and coatingscomprising a doped VO₂ absorbing layer, as shown in FIGS. 14a and 14 b,a source of heat needs to be incorporated in the apparatus. Aconventional spectrophotometer with a heating attachment may be used toprovide both the means for heating the coating and for detectingreflected radiation in a single unit. An infrared imager which couldvisually display the contrast change may also be used to detectradiation reflected from the coating.

Conventionally, the infrared wavelength region is taken to meanwavelengths between 700 nm and 1000 μm. For the purpose of thisspecification the infrared wavelength region of interest is between 700nm and 15 μm, and preferably between 700 nm and 12 μm.

What is claimed is:
 1. A multi layer interference device having areflectance spectrum comprising at least one maximum comprising; atleast one multilayer stack, a reflective layer having at least onesurface for carrying said at least one multi layer stack, said at leastone multi layer stack comprises a first layer of dielectric material, alayer of absorbing material and a second layer of dielectric materialarranged in series with the layer of absorbing material situated betweenthe first and second layers of dielectric material, wherein the secondlayer of dielectric material has substantially the same opticalthickness as the first layer of dielectric material at a wavelengthsubstantially corresponding to a maximum in the reflectance spectrum andwherein the layer of absorbing material has a refractive index n and anoptical constant k, such that incident electromagnetic radiation, havinga wavelength at which odd multiples of half wavelengths substantiallycorrespond to the optical thickness of the at least one multi layerstack at said wavelength, is substantially absorbed within the device,wherein the layer of absorbing material is a conductivesubstoichiometric metal oxide and in that the device has a reflectancespectrum in the infrared wavelength region.
 2. The device of claim 1wherein the first dielectric material is the same as the seconddielectric material.
 3. The device of claim 1 wherein at least one ofthe first or second dielectric materials is any one of titanium oxide(TiO₂), magnesium fluoride (MgF₂), zinc sulphide (ZnS), zinc selenide(ZnSe), silicon (Si), germanium (Ge) or barium fluoride (BaF₂).
 4. Thedevice of claim 3 wherein the n/k ratio of the absorbing material isbetween 0.7 and 1.3 in the infrared wavelength region.
 5. The device ofclaim 4 wherein the n/k ratio of the absorbing material is substantiallyequal to 1 in the infrared wavelength region.
 6. The device of claim 5wherein the reflective layer is a metal.
 7. The device of claim 5wherein the reflective layer is any one of a conducting oxide, aconducting nitride, a conducting silicide or a conducting sulphide. 8.The device of claim 1 wherein the substoichiometric metal oxideabsorbing material is of the same material as the layer of dielectricmaterial.
 9. The device of claim 8 wherein the substoichiometric metaloxide is TiO_(x) and the dielectric material is TiO₂.
 10. The device ofclaim 1 wherein the reflective layer has two opposite facing surfaceswherein at least one multi layer stack is deposited on each of the twoopposite facing surfaces such that the device has a substantiallysymmetric structure about the reflective layer.
 11. The device of claim1 wherein the device is in the form of a thin film.
 12. The device ofclaim 11 wherein the thin film is flaked into fragments.
 13. A mouldedarticle incorporating the fragments of thin film of the device accordingto claim
 12. 14. A paint or ink incorporating the fragments of thin filmof the device according to claim
 12. 15. An article, wherein said paintor ink of the device according to claim 14 is applied to the surface ofsaid article.
 16. An article having a label, said label having saidpaint or ink of the device according to claim 14 applied thereto. 17.The device of claim 1 wherein the reflective layer is a reflectingparticulate substrate.
 18. The device of claim 17 wherein theparticulate substrate is substantially spherical.
 19. The device ofclaim 1 wherein the reflective layer is deposited on a substantiallynon-reflecting particulate substrate.
 20. A system for marking anarticle and checking its authenticity comprising; the device of claim 1wherein the device is applied to the article to be authenticated, anilluminator for illuminating the device with incident radiationcomprising one or more wavelengths wherein one or more of thewavelengths substantially correspond to a maximum or a minimum in thereflectance spectrum of the device and a detector for detectingradiation reflected from the device at one or more of the wavelengths,whereby the detection of the reflected radiation provides an indicationof the authenticity of the article.
 21. The system of claim 20, wherebya comparison of the reflected radiation at two or more wavelengthsprovides an indication of the authenticity of the article.
 22. Thesystem of claim 20 wherein the detector for detecting infrared radiationreflected from the device is a thermal imager.
 23. The system of claim20 wherein the detector for detecting infrared radiation reflected fromthe device is a spectrophotometer.
 24. A multi layer interferencedevice, having a reflectance spectrum in the infrared wavelength regioncomprising at least one maximum, comprising; at least one multilayerstack, a reflective layer having at least one surface for carrying saidat least one multi layer stack, wherein said at least one multi layerstack comprises a first layer of dielectric material, a layer ofabsorbing material and a second layer of dielectric material arranged inseries with the layer of absorbing material situated between the firstand second layers of dielectric material, wherein the second layer ofdielectric material has substantially the same optical thickness as thefirst layer of dielectric material at a wavelength substantiallycorresponding to a maximum in the reflectance spectrum and wherein thelayer of absorbing material has a refractive index n and an opticalconstant k, such that incident electromagnetic radiation, having awavelength at which odd multiples of half wavelengths substantiallycorrespond to the optical thickness of said at least one multi layerstack at said wavelength, is substantially absorbed within the deviceand wherein the layer of absorbing material is any one of a conductingoxide, a conducting nitride, a conducting silicide or a conductingsulphide for which the n/k ratio is substantially equal to 1 in theinfrared wavelength region.
 25. The device of claim 24 wherein thereflective layer is a metal.
 26. The device of claim 24 wherein thereflective layer is any one of a conducting oxide, a conducting nitride,a conducting silicide or a conducting sulphide.
 27. The device of claim24 wherein the absorbing material is substantially the same material asthe reflective layer.
 28. The device of claim 24 wherein the absorbingmaterial is any one of indium tin oxide (ITO), doped tin oxide ortitanium nitride (TiN).
 29. The device of claim 24 wherein the layer ofabsorbing material is vanadium dioxide (VO₂) and the reflectancespectrum of the device may be varied with temperature.
 30. A system forcovertly marking an article and checking its authenticity comprising;the device of claim 29, wherein the device is applied to the article tobe authenticated; a temperature control for varying the temperature ofthe device such that the reflectance of the device at one or morewavelengths may be varied as the temperature is varied; an illuminatorfor illuminating the device with infrared radiation comprising one ormore wavelengths substantially corresponding to one or more of thewavelengths at which the reflectance varies and a detector for detectinginfrared radiation reflected from the device at one or more wavelengthsat which the reflectance varies, whereby a comparison of the reflectedradiation before and after the temperature of the device is variedprovides an indication of the authenticity of the article.
 31. Thesystem of claim 30 wherein the means for illuminating the device and thetemperature control is a single laser.
 32. The device of claim 24wherein the absorbing material is substoichiometric vanadium dioxide(VO_(2−x)) and the reflectance spectrum of the device may be varied withtemperature.
 33. The device of claim 24 wherein the absorbing materialis doped vanadium dioxide and the reflectance spectrum of the device maybe varied with temperature.
 34. A multi layer interference device,having a reflectance spectrum in the infrared wavelength regioncomprising at least one maximum, comprising: at least one multilayerstack, a reflective layer having at least one surface for carrying saidat least one multi layer stack, said at least one multi layer stackcomprises a layer of dielectric material and a layer of absorbingmaterial, wherein the layer of absorbing material has a refractive indexn and an optical constant k, such that incident electromagneticradiation, having a wavelength at which odd multiples of quarterwavelengths substantially correspond to the optical thickness of said atleast one multi layer stack at said wavelength, is substantiallyabsorbed within the device, wherein the layer of absorbing material is anon-metal absorbing material.
 35. The device of claim 34 wherein thereflective layer is a metal.
 36. The device of claim 34 wherein thereflective layer is any one of a conducting oxide, a conducting nitride,a conducting silicide or a conducting sulphide.
 37. The device of claim34 wherein the layer of non-metal absorbing material is any one of aconducting oxide, a conducting nitride or a conducting silicide forwhich the n/k ratio is substantially equal to 1 in the infraredwavelength region.
 38. The device of claim 37 wherein the non-metalabsorbing material is any one of ITO, doped tin oxide or TiN.
 39. Thedevice of claim 37 wherein the non-metal absorbing material is vanadiumoxide (VO₂) and wherein the reflectance spectrum may be varied withtemperature.
 40. The device of claim 37 wherein the non-metal absorbingmaterial is substoichiometric vanadium oxide (VO₂) and wherein thereflectance spectrum may be varied with temperature.
 41. The device ofclaim 37 wherein the non-metal absorbing material is doped vanadiumoxide and wherein the reflectance spectrum may be varied withtemperature.
 42. The device of claim 37 wherein the non-metal absorbingmaterial is a substoichiometric metal oxide.
 43. The device of claim 42wherein the substoichiometric metal oxide is TiO_(x).
 44. A multi layerinterference device, having a reflectance spectrum in the visiblewavelength region comprising at least one maximum, comprising; at leastone multilayer stack, a reflective layer having at least one surface forcarrying said at least one multi layer stack, said at least one multilayer stack comprises a layer of dielectric material and a layer ofabsorbing material, such that incident electromagnetic radiation, havinga wavelength at which odd multiples of quarter wavelengths substantiallycorrespond to the optical thickness of said at least one multi layerstack at said wavelength, is substantially absorbed within the device,wherein the absorbing material is a conducting substoichiometric metaloxide.